The present invention relates generally to variable geometry turbines and, more particularly, to variable geometry turbines for internal combustion engine turbochargers, capable of operating efficiently over a broad range of engine speeds.
Turbochargers used on internal combustion engines can be designed to operate efficiently at a particular engine load and speed. Difficulties are encountered, however, when turbochargers are required to operate over a broad range of engine speeds, such as experienced with engines used in heavy-duty trucks. Turbochargers and other fixed geometry turbines operate at their maximum efficiency at the particular flow and pressure values for which they are designed. When they are operated at other than the conditions for which they are designed, however, losses are incurred which cause decreased efficiency in both the compressor component and the turbine component of the turbochargers.
If a turbocharger is designed to operate best on an internal combustion engine operating at high speeds (i.e. full throttle and high engine r.p.m.), the turbocharger will be considerably off its maximum efficiency point when the engine is "torqued down" to lower engine opeaating speeds. Conversely, if a turbo-charger is designed to operate most efficiently with the internal combustion engine operating in its low speed, torque peak operating range, the turbocharger will overspeed considerably when the engine is operated under full throttle, high engine speed conditions. Thus, turbocharger efficiency must be seriously compromised in applications such as truck engines which must operate both at high speeds on level freeway type roads and at torque peak operating speeds in pulling heavy loads over mountainous terrain.
To prevent overspeed in turbochargers which are matched to an engine at its low speed, torque peak operating range, a waste gate is frequently provided to bleed off exhaust gas to atmosphere ahead of the turbine to prevent the maximum speed of the turbocharger from exceeding safe limits when the engine is operated at high speed. The use of waste gates, however, allow energy to escape from the turbocharger system, resulting in a substantial loss in turbocharger efficiency.
The benefits obtainable with variable geometry systems are well known. Such benefits include lower engine fuel consumption, quicker acceleration of the turbocharger-engine combination in response to load and speed changes, less smoke in the engine exhaust during acceleration under load, and high torque at low engine speed while preventing turbocharger overspeed at a high engine speed and load.
Because of these benefits, there have been many attempts to design and develop variable geometry turbine components for turbochargers. Many such prior efforts involved employing variably positioned nozzle vanes in the turbine. In these prior systems the position of the nozzle vanes determined the nozzle area, and, the position of the nozzle vanes was varied as the engine load and speed changed in an attempt to match efficient turbocharger operating conditions with the engine operating conditions. These prior devices were complicated and expensive. They required rather small vane elements to be connected together in a manner which permitted them to change position precisely in conjunction with one another and with close clearances between their stationary and moving surfaces so that gas leakage was minimized. If any significant leakage occurred from one nozzle to another, the efficiency of the turbine suffered considerably. They were also required to function in the hottest portion of the exhaust gas flow path. Since the variable turbine components functioned in the hot engine exhaust gas environment, they are subject to distortion and cracking and subsequent failure.
In addition, the prior variable geometry turbine systems required a control system to position the variable components in specific locations dictated by the load and speed demands of the engine. The control system and the moving components contributed significantly to the cost of the engine-turbocharger combination. To date, the high cost of such variable geometry systems and the reliability problems of the complicated mechanisms that they employ have prevented a commercially successful variable geometry system from being developed.